This invention relates to methods of producing etched patterns in metal bodies. More particularly, it is concerned with a method of fabricating color television picture tube aperture masks.
In color television picture tubes, the beams of electrons emanating from the cathodes are directed by means of electromagnetic deflection coils through the holes of an aperture mask and onto a pattern of color generating phosphors deposited on the inner surface of the picture tube face panel. The aperture mask may contain upwards of a quarter million holes which are formed by photoresist etching processes well known in the art.
In a typical process for forming a picture tube aperture mask, a sheet of thin metal which is usually unalloyed steel is first coated with a film of water soluble photoresist material. The film is then exposed through a photographic master to produce a latent aperture pattern in the film. The exposed film is developed with water to remove the unexposed photoresist. The photoresist remaining on the steel surface is next hardened to attack by aqueous acid by heating. The aperture pattern is then etched in the steel mask blank by exposing it to an acid etching bath. Following the etching step, the photoresist film remaining on the metal substrate is removed by means of an aqueous caustic stripping step.
A photoresist material to be compatible with the process described above must be (1) water soluble, (2) insolubilized to attack by water upon exposure to light, (3) insolubilized to attack by aqueous acid etching baths by heat hardening, and (4) soluble in aqueous caustic solutions after the heat hardening step.
The photoresist material most commonly employed for manufacture of picture tube aperture masks is so-called "fish glue" sensitized to light by the addition of ammonium dichromate. Fish glue is a partially hydrolyzed protein polymer derived from waste by-products of the fish processing industry. It is commercially available as an aqueous solution which is about 50% solids by weight. Fish glue varies widely from batch to batch in such properties critical to its use as a photoresist material as viscosity, drying characteristics, and response to light once sensitized. This variation in critical properties presents major problems when fish glue photoresist formulations are employed in automated or assembly line processes for aperture mask fabrication.
Dichromated polyvinyl alcohol photoresists, well known to the art, would appear to offer an attractive alternative to fish glue as a photoresist material for aperture mask production. Polyvinyl alchol, in addition to possessing most of the desired photoresist properties outlined above, is a synthetic polymer commercially available in a variety of molecular weight grades which are both uniform and consistent from one batch to the next. As a photoresist material, dichromated polyvinyl alcohol does possess one property which, nevertheless, precludes its use in existing processes for aperture mask fabrication. Heat hardened films of dichromated polyvinyl alcohol are very difficult to remove from metal surfaces using aqueous caustic stripping solutions. These films are also not easily stripped by organic solvents which, in any event, are less desirable because of their cost and associated disposal problems. Alternative stripping solutions which have been found to be effective are generally corrosive to the metal mask itself or to metallic parts of the stripping machinery.
In copending application Ser. No. 951,922 entitled "Photoresist and Method for Color Television Picture Tube Aperture Mask Production" filed concurrently herewith and assigned to the assignee of the present invention, there is described a method of overcoming the problem of the resistance to stripping of polyvinyl alcohol photoresist films by the modification of the photoresist formulation with polymeric additives.
It is therefore an object of this invention to provide a process for fabricating etched metal bodies which employs water soluble photoresist formulations which are uniformly compoundable from batch to batch.
It is a further object of this invention to provide a process for fabricating color television picture tube aperture masks which allows for the use of unmodified polyvinyl alcohol photoresist formulations.
It is still a further object of this invention to provide a process for fabricating color television picture tube aperture masks which is compatible with existing process machinery.